1. Field of the Invention
The present invention relates to an electrical filter circuit for processing analog sampling signals and more particularly to a filter circuit consisting of a plurality of capacitors with switching means for interconnecting the capacitors with an operational amplifier and a reference potential at predetermined clock times.
2. The Prior Art
Switched capacitor filters, employing plural capacitors and operational amplifiers have been described in "Switched-Capacitor Filter Design Using the Bilinear z-Transform", in "IEEE Transactions on Circuits and Systems" Volume CAS-25, No. 12, December 1978, pp. 1039-1044, and in "Switched-Capacitor Circuits Bilinearly Equivalent to Floating Inductor or F.D.N.R." in "Electronics Letters", Feb. 1, 1979, Volume 15, No. 3, pp. 87-88. Such filters do not process analog signals on a continuous basis, but instead process discrete time samples generated at the frequency F, with a sampling period of T=1/F. A variety of suitable sampling circuits are known in the art. Circuit means is necessary for connecting a sampling circuit between an analog signal source and a filter circuit, and also for reconverting the signals available at the output filter into continuous analog signals. The significant feature of switched capacitor filters is their simulation of inductances entirely by means of active circuit elements and capacitors, so that they are suitable for integration in monolithic form. Switched capacitor filters employ operational amplifiers and it is desirable to attain the smallest possible total number of circuit elements, while maintaining stability of the circuits.
In known circuits for switched capacitor filters, the inverting input is connected by way of a capacitor to the output of an operational amplifier, to provide in effect a counter coupling. However, the operational amplifiers typically employed require a high common mode rejection, since the non-inverting input of the operational amplifier cannot always be held at ground potential. In addition, when the capacitors are formed using MOS (metal oxide silicon) technology, the capacitance to ground unavoidably accompanying floating MOS capacitors can lead to significant disruptions of the filter function.
Circuit designs for switches used in connection with switched capacitor filters are known in the art. They are typically designed in such a way so they can be formed by an integration process together with the remaining filter structures. MOS transistors are particularly suitable for such switches. Such switches have control electrodes which, by application of a control voltage, render the switches conductive. When the control voltage is absent, the switch represents an open circuit, or an electrical interruption.